Transcript The Transition Zone: Slabs’ Purgatory

The Transition Zone:
Slabs’ Purgatory
CIDER, 2006 - Group A
Garrett Leahy, Ved Lekic, Urska Manners,
Christine Reif, Joost van Summeren, Tai-Lin
Tseng, Magali Billen, Wang-Ping Chen,
Adam Dziewonski
Tonga Seismicity
Predicted Slab Positions
Degree 45 and 24
spherical
harmonic
expansions of
locations of
slabs based on
plate history
reconstructions
assuming no
stagnation in
transition zone.
Tomographic Models
Harvard
Berkeley
Preliminary Conclusions
• Tomography reveals larger fast regions in the western Pacific
transition zone.
• Deep earthquake stress axes show evidence of resistance to
crossing the 660 km discontinuity.
• Structure below and above 660 km discontinuity has different
spectral character.
• Implication: slabs stagnate in the transition zone for some length
of time.
A Simple Force Balance for
slabs in the Transition Zone
x
z
Fb =∫ gdxdz

Constraints on  and Clapeyron slopes
• Density contrasts
– Seismic constraints
– Lab experiments on mantle minerals/rocks
– Lattice dynamics simulation
• Clapeyron slopes
– Lab experiments on phase transformation
– Calorimatric Calculations
Summary Phase Transition
Data
Density Contrast
Seismic Constrains
Calculations
(Pyrolite)
410
5% to 6%
About 3%
660
7% to 9%
6% to 7%
Simulations
(MgSiO3)
About 8%
Clapeyron Slope
Lab Experiments
dP/dT 410 (Mpa/K)
 to b
2.5 to 4
dP/dT 660 (Mpa/K)
g to Mw+Pv
–3 to –1
dP/dT 660 (Mpa/K)
Pyrolite
-0.5
Calorimatric Calculation
About -3
For Clapeyron Slope of Olivine Polymorphs: Duffy, T., Synchrotron facilities and the study of the Earth's deep interior.
Rep. Prog. Phys. 68 (2005) 1811-1859.
Slab Thermal Anomaly
Gaussian
Cross-slab
Profile
Max. Slab
Depth:
1000 km
Exponential
Decrease
In Peak
Anomaly
Max. Slab
Depth:
500 km
Phase Transition Anomaly
410: g = 3.0 MPa/K
660: g = -1.3 MPa/K
 = 3-6%
 = 7-9%
Temperature Anomaly
410:
g= 4.0 MPa/K
 = 4%
660:
g= -2 MPa/K
 = 3%
Transition Height (km)
Total Force (x 1012 N/m)
Effect of Dip on Sum of Thermal
And Phase Change Forces
16
12
8
4
0
0 10 ---Dip (degrees)-- 80 90
Change in Density at 410 (%)
Effect of Density Change at Phase
Boundaries
6
5.
4
3
6 6.5 7 7.5 8 8.5 9
Change in Density at 660 (%)
Clapeyron Slope at 410 Mpa/K
Effect of Clapeyron Slope
5
4
3
2.5
-3
-2
-1
-0.5
Clapeyron Slope at 660 Mpa/K
Effect of Shear Forces
um=1019 Pas
tran = 1020 Pas
Major slowing
occurs upon
entering
lower mantle
Lower mantle
viscosity greater
than 1022 Pa s
can strongly hinder
Slab.
Metastable Olivine
Growth Rate:
G(T) = A*k*T*exp[-H/(RT)](1-exp[G/(RT)])
k=exp(10) Growth constant
A = 1e-3
Extrapolation parameter for
low T in slab.
Depth of Metastable Olivine in Slab
z ~v*ln(1-f)/(-2*S*G)
v
Slab velocity
S = 1/d
Grain boundary Surface Area/Volume
f = 0.95
Volume fraction of wadsleyite at
completion of transformation.
Cooler Temperature strongly inhibits transformation.
What about a Metastable Olivine
Wedge?
Conclusions
• Buoyancy from temperature can be order of magnitude larger than
other forces.
– Need dynamic model of temperature.
• Extra buoyancy from 410 phase change may be much larger than
resisting buoyancy from 660.
• Shear forces beneath 660 may significantly hinder slab sinking into
lower mantle.
• If phase parameters at 410 and 660 are comparable, then a moderately
high viscosity in lower mantle can hinder slab.
• If metastable olivine exists, it can “easily” stop slabs in the transition
zone, especially for large grain size (~ cms)